The cryogenic separation of mixtures such as air to produce nitrogen is a well established industrial process. Liquid and vapor are passed in countercurrent contact through one or more columns of a cryogenic rectification plant and the difference in vapor pressure between the oxygen and nitrogen causes nitrogen to concentrate in the vapor and oxygen to concentrate in the liquid. The lower the pressure is in the separation column, the easier is the separation due to vapor pressure differential. Accordingly, the final separation for producing product nitrogen is generally carried out at a relatively low pressure.
Often the product nitrogen is desired at a high pressure. In such situations, the product nitrogen is compressed to the desired pressure in a compressor. This compression is costly in terms of energy costs as well as capital costs for the product compressors.
Another way of producing high pressure nitrogen product is to operate the column or columns of the cryogenic air separation plant at an elevated pressure. This is disadvantageous because it makes the separation more difficult for any desired product purity level and also increases the burden on the base load air compressor which initially processes the feed air thus increasing the operating costs of the process.
Accordingly, it is an object of this invention to provide a cryogenic rectification system wherein product nitrogen may be efficiently produced while avoiding high operating pressures within the cryogenic rectification plant thus not burdening the base load air compressor with such high operating pressures.